In recent years, a gallium nitride-based compound semiconductor represented by the formula AlxInyGa1-x-yN (0≦x<1, 0≦y<1, x+y<1) has attracted much attention as a material for a light-emitting diode (LED) emitting ultraviolet to blue light, or green light. Light emission at high intensity in the ultraviolet and blue region and in the green region, which was hitherto difficult, has been made possible by using a semiconductor made of these materials. Gallium nitride-based compound semiconductors are generally grown on a sapphire substrate. As this is an insulating substrate, unlike that for GaAs-based light-emitting devices, an electrode cannot be provided on rear surface of the substrate. Therefore, both negative and positive electrodes must be provided on the semiconductor grown as a crystal.
In particular, in the case of a semiconductor device using a gallium nitride-based compound semiconductor, as the sapphire substrate can transmit light at the wavelength of the emitted light, a flip-chip-type structure, in which the device is mounted with the electrode surface on the underside, and light is extracted from the side of the sapphire substrate, has attracted much attention.
FIG. 1 is a schematic view showing an example of general structure of light-emitting device of this type. Thus, a light-emitting device has a buffer layer 2, a n-type semiconductor layer 3, a light-emitting layer 4, and a p-type semiconductor layer 5 successively grown as crystal on a substrate 1, with a portion of the light-emitting layer 4 and the p-type semiconductor layer 5 removed by etching so as to expose the n-type semiconductor layer 3, and a positive electrode 10 is formed on the p-type semiconductor layer 5 and a negative electrode 20 is formed on the n-type semiconductor layer 3. Such a light-emitting device is mounted, for example, with the surface having electrode formed thereon facing a lead frame, and then is bonded. Light emitted from the light-emitting layer 4 is extracted from the side of the substrate 1. In order to extract light efficiently in this type of light-emitting device, a reflective metal is used as the positive electrode 10, and is provided so as to cover the major portion of the p-type semiconductor layer 5 to thereby cause the light from the light-emitting layer toward the positive electrode to be reflected by the positive electrode 10 and to be extracted from the side of the substrate 1.
Accordingly, it is necessary that material for the positive electrode has a low contact resistance with the p-type semiconductor layer and a high reflectance. Generally, it has been known that silver (Ag) has the highest reflectance. It has also been known, however, that Ag often causes electromigration to occur.
Electromigration is a phenomenon in which a material is ionized and diffuses in the presence of water. For example, if electric current is supplied, in an atmosphere wherein water exists, to the electrode in which Ag is used, a deposit mainly composed of Ag is formed. If the deposit generated from the positive electrode reaches a negative electrode or the deposit couples a p-type semiconductor layer with an n-type semiconductor layer, the inverse voltage lowers to deteriorate the characteristic of the light-emitting device with time. Accordingly, when Ag is used as a reflective positive electrode, it is necessary to restrict the electromigration of Ag so that the characteristic is stabilized.
Japanese Unexamined Patent Publication (Kokai) Nos. 11-186598 and 11-186599 propose means for using Ag as material of the positive electrode, wherein a silver layer is provided on a p-type nitride semiconductor layer and a stabilized layer is further provided on the silver layer. It is described that the stabilized layer serves to facilitate the mechanical and electrical characteristic of the silver layer. The Ag layer could be formed in a stable manner on the p-type nitride semiconductor layer by controlling a deposition rate of Ag layer and a temperature of a sapphire substrate during the deposition.
Also, in Japanese Unexamined Patent Publication (Kokai) No. 11-220171, there is a proposal in that a first metallic layer (Ag) connected to the p-type semiconductor layer is covered with a second metallic layer. It is described that no electromigration occurs since the second metallic layer covers the Ag layer.
The provision of the stabilized layer on the silver layer, however, results in such problems as the rise of production cost and/or complication of the production process.